The lesser-spotted butterfly

Small tortoiseshells are popular visitors to gardens across the country – or at least they used to be. Now they have disappeared, and nobody is quite sure why.

Gone from your garden: one of Britain's best-loved and most attractive butterflies, and formerly, one of the commonest. The small tortoiseshell, for reasons no one really understands, has declined so rapidly that in just a few years it has become a threatened species.

Since 1990, numbers of this most attractive of insects have fallen by 58 per cent across Britain, and in some parts of the country, the picture is far worse: in south-east England, over that period, its population has plummeted by 82 per cent, and the plunge may be continuing. Data show that 2006 and 2007 were the worst since counting began in 1976, and followed a bad year in 2005.

Butterfly Conservation, the charity which yesterday released details of the small tortoiseshell's alarming drop in population, says recorders in the South-east have been extremely worried over the past few years and report that the butterfly is no longer one of their most frequently seen garden visitors. "They used to be the most common butterfly in my garden," says Tony Hoare, a butterfly enthusiast from Surrey. "Now they have virtually disappeared."

Scientists are struggling to find out just what is happening to Aglais urtica. The scientific name means the nettle beauty, referring to the insect's food plant, stinging nettles. It is apt: the pretty wing pattern of orange, yellow, white and black is set off by small spots of brilliant blue at the wing edges.

With the losses greater in south-east England, Butterfly Conservation says that suggests the problem may be linked to climate change, because climate warming is more advanced in the South-east. But if there is a link, the precise mechanism remains unclear.

Another suspect is a tiny parasitoid fly, Sturmia bella, first noted in Britain in 1999. Its arrival from southern Europe is either itself the result of climate change, or an unwitting introduction from across the Channel.

The fly has developed a cunning strategy to infect its host, laying its eggs in the leaves of the nettles on which the small tortoiseshell caterpillars feed. The caterpillars ingest the eggs with their food, whereupon the eggs hatch and develop within the caterpillar, eventually killing it and hatching just when it becomes a chrysalis. "Eventually little flies burst out of the cocoon," in the manner seen in the film, Alien," Butterfly Conservation reports.

It is known that where there is infection, the parasite can devastate tortoiseshell populations, and Butterfly Conservation has launched a research project with Oxford University's Department of Zoology to determine the extent, if any, that the fly is to blame. Hundreds of volunteers are expected to help the Oxford research team by collecting batches of caterpillars.

But essentially, the decline remains a mystery. It is all the more so in that, although more than 70 per cent of Britain's 55 resident butterfly species have also declined in the past 25 years, the mechanism in almost every case is clear, and is usually connected to the loss of a particular species' habitat.

The reasons are known in certainly the four most rapidly declining species, the high brown fritillary (79 per cent decline); the wood white (65 per cent); the pearl-bordered fritillary (61 per cent) and the Duke of Burgundy (52 per cent decline).

For the two fritillaries, which feed on violets in woods, it is the disappearance of coppicing, the ancient practice of woodland thinning which has now virtually stopped. For the wood white, which feeds on vetches and other leguminous plants in rides and on woodland edges, its specialised areas may have become overgrown. For the Duke of Burgundy, it is the disappearance of grazing on chalk downland.

But nothing major has happened – apparently – to the habitat of the tortoiseshell, or its food plant, stinging nettles. Indeed, three closely related butterflies which also feed on nettles are doing very well. They are fellow members of the Vanessae family, the red admiral, the comma and the peacock. The peacock's population is stable, but both the red admiral and the comma are expanding in numbers and in distribution through Britain, moving steadily northwards. "The case of the small tortoiseshell is a real mystery," said Butterfly Conservation's chief executive, Dr Martin Warren. "This is an iconic butterfly, known and loved by very many people, and we have been shocked at the massive decline. We are desperate to find the causes.

"Butterflies are great indicators, telling us much about climate change and the state of the environment. But there are times like this when we need experts to interpret just what they are telling us. In this case, there may be a serious message about unexpected consequences of global warming."

Butterflies and moths need warmth to function as adults, so climate change might be expected to benefit them, but that can have perverse results. For example, it has been found that the similarly handsome garden tiger moth is dropping in numbers because it overwinters as a caterpillar, and the caterpillars are not suited to the increasingly warm and damp winters that Britain is experiencing.

The most striking aspect of the small tortoiseshell decline is just how familiar and common an insect it previously was. It is not quite the house sparrow of the butterfly world – that would be the large or the small white – but in bird terms, it is certainly the insect equivalent of the chaffinch, similarly colourful, well-known and well-liked.

Its decline has been picked by the UK Butterfly Monitoring Scheme, which is run jointly by Butterfly Conservation and the Government's Centre for Ecology and Hydrology, and is the largest such scheme anywhere in the world. It involves more than 1,000 volunteer recorders making regular counts at more than 800 sites across the UK.

Butterflies are declining far faster than other well-known groups such as birds and plants because they are highly sensitive to environmental change, and may give advance warning of dangers to other wildlife.

Most rapidly declining butterfly species

High brown fritillary ( Argynnis Adippe)

This large, powerful butterfly has undergone a 79 per cent decline in the past 25 years. Once widespread in England and Wales, it is now reduced to around 50 sites on Dartmoor and Morecambe Bay, and in a few locations in western England and in Wales, where conservationists are working to save it from extinction. It is usually seen flying swiftly over the tops of bracken or low vegetation in woodland clearings. In flight, the males are almost impossible to separate from those of the dark green fritillary, which often share the same habitats. However, both species frequently visit flowers such as thistles and bramble where it is possible to see their distinctive underside wing markings.

Wood white ( Leptidea sinapis)

A small butterfly with a dainty style of flight, usually found in woodland glades or scrub, which has undergone a 65 per cent decline. It breeds in tall grassland or light scrub in partially shaded or edge habitats. In Britain, most colonies breed in woodland rides and clearings, though a few large colonies occur on coastal undercliffs. A few smaller colonies occur on disused railway lines and around rough, overgrown field edges (for example in north Devon). In Ireland, more open habitats are used, often far from woodland, including rough grassland with scrub, road verges, hedges, and disused railway lines.

Pearl-bordered fritillary ( Boloria euphrosyne)

Medium-sized, orange and black with silver markings on the underside, this butterfly was once widespread. It is now highly threatened in Englandand Wales, having dropped in numbers by 61 per cent. There are scattered colonies in England and Wales, although it is more widespread in Scotland, and it is also found in the Burren region of Ireland. This is one of the earliest fritillaries to emerge and can be found as early as April in woodland clearings or rough hillsides with bracken. It flies close to the ground, stopping regularly to feed on spring flowers such as bugle.

Duke of Burgundy ( Hamearis lucina)

This small butterfly, orange and brown, like a tiny fritillary, frequents scrubby grassland and sunny woodland clearings, typically in very low numbers and only in England, where its stronghold is in central-southern areas, with more isolated colonies in the southern Lake District and North Yorkshire. In recent years it has declined by 52 per cent. The adults rarely visit flowers and most sightings are of the territorial males as they perch on a prominent leaf at the edge of scrub. The females are elusive and spend much of their time resting or flying low to the ground looking for suitable egg-laying sites.